The present invention relates to an apparatus for controlling a hydraulic pressure used for speed change performed by an automatic transmission of an automobile, and particularly to a hydraulic control apparatus for an automatic transmission which is intended to directly control a hydraulic pressure to be supplied to a clutch on the basis of an electric signal.
The control apparatus of this type has been known, for example, from Nissan's maintenance guide titled "Full Range Electronically Controlled Automatic Transmission" issued by NISSAN MOTOR on March, 1987. In this control apparatus, an accumulator is disposed between a manual spool valve and a forward clutch and another accumulator is disposed between the above spool valve and a reverse clutch. Another known hydraulic control apparatus generates a variation in torque of an output shaft of an automatic transmission when the position of a shift lever of an automobile is changed between an N (Neutral) range and a D (Drive: forward) range or between the N range, and an R (Reverse) range is relaxed by mechanically and electrically controlling a line pressure introduced in the above spool valve. At the present time, the hydraulic control apparatus having such a structure has been extensively used for automobiles
The above-described known hydraulic control apparatus, however, is disadvantageous in that provision of two accumulators for forward and reverse clutches increases the size of a transmission using the control apparatus, and the accuracy of control for suppressing a variation in torque is deteriorated due to changes in mechanical components with time.
An object of the present invention is to provide a hydraulic control apparatus for an automatic transmission, which is improved in accurately suppressing a variation in torque generated when the position of a shift lever of an automobile is changed between an N range and a D range or between the N range and an R range and in preventing the accuracy of control for suppressing a variation in torque from being deteriorated due to changes of mechanical components with time, by adopting a control system using a perfectly electronic circuit in place of a control system using a hydraulic circuit.
To achieve the above object, according to the present invention, a hydraulic control apparatus for an automatic transmission having apparatus for generating a hydraulic pressure by driving a hydraulic pump using an engine drive force or an electric force; a spool valve automatically or manually driven by operation of a speed change lever; at least one clutch used for forward movement, reverse movement or speed change of a vehicle; a hydraulic control actuator for governing a hydraulic pressure applied to the clutch; and a control for the hydraulic control actuator. A pressure is provided for a hydraulic pressure which is supplied from the hydraulic pressure generating apparatus to the clutch through the spool valve, by the hydraulic control actuator to suppress a variation in output torque of the automatic transmission. In the above hydraulic control apparatus, preferably, the hydraulic pressure governed by the actuator is supplied to the forward clutch and the reverse clutch through a hydraulic line switching apparatus for switching hydraulic lines communicated to the forward clutch and the reverse clutch from each other.